1. Field of the Invention
The present invention relates to a fabrication method of an epilayer structure for InGaAsP/InP ridge waveguide phase modulator with high phase modulation efficiency. In more detail, it relates to a P-p-n-N InGaAsP/InP double heterostructure (DH) ridge waveguide phase modulator, fabricated to be that phase change of the TE-mode is linearly proportional to reverse bias voltage at 1.55 μm wavelength, having high phase modulation efficiency.
2. Description of the Related Art
III–V compound semiconductors enable monolithic integration, which is unable in LiNbO3, to save the expense and improve the reliability. In addition, on the contrary to an LiNbO3 optical modulator simply using electro-optic effect for changing the refractive index of a waveguide, a optical modulator using III–V compound semiconductors can use free-carrier effect as well as electro-optic effect at the same time, and thus the larger change of the refractive index can be obtained. Consequently, the larger phase change can be obtained with low voltage.
As the refractive index of III–V compound semiconductor is changed, a phase modulation of the propagating optical wave is being occurred, and thus a phase modulator can be achieved. If a phase modulator is being combined with a Mach-Zehnder (MZ) interferometer or a directional coupler structure, an amplitude modulation and/or a switching can be obtained. Therefore, a phase modulator, among the various kinds of optical modulators, is a key external modulator for a high-speed optical communication and/or an optical signal processing system.
Recently, a waveguide phase modulator and a switch, based on GaAs and InP, are being popularly developed. Looking into the phase modulators based on GaAs, it is reported that, in a P-p-n-N GaAs/AlGaAs double heterostructure (DH) phase modulator, which increases the overlapping of the guided mode field and the applied electric field compared with the conventional P-n-N structure and takes advantages of electric field effect and free-carrier effect as well, the phase modulation efficiencies are measured to be 96°/V·mm at 1.06 μm wavelength [G. Mendoza-Alvarez, L. A. Coldren, A. Alping, R. H. Yan, Hausken, K. Lee, and K. Pedrotti, IEEE J. Lightwave Technol. LT-6, 798 (1988)] and 48.9°/V·mm at 1.31 μm wavelength[Y. Byun, K. Park, S. Kim, S. Choi, Y. Chung, and T. Lim, Ungyong Mulli (The Korean Physical Society), 10, 101 (1997)] respectively. Besides, a P-P-p-i-n-N-N GaAs/AlGaAs W-waveguide phase modulator shows the phase modulation efficiency of 34.6°/V·mm and the propagation loss of 0.2–0.6 dB/cm at 1.31 μm wavelength, which are the best characteristics reported up to the present[Y. Byun, K. Park, S. Kim, S. Choi, J. Yi, and T. Lim, Appl. Opt., 37, 496 (1998)].
On the other hand, looking into the phase modulators based on InP, a P-i-n-N InGaAsP/InP phase modulator, which has double heterostructure, shows the phase modulation efficiency of 11°/V·mm for the TE mode at 1.52 μm wavelength [J. F. Vinchant, J. A. Caviles, M. Eman, P. Jarry, and M. Renaud, IEEE J. Lightwave Technol., 10, 63 (1992)]. And an InGaAlAs/InP rib waveguide phase modulator, which has single heterostructure, shows comparatively low phase modulation efficiency of 5.5°/V·mm for the TE mode at 1.3 μm wavelength [S. -K. Han, R. V. Ramaswamy, W. -Q. Li, and P. K. Bhattacharya, IEEE Photon. Technol. Lett., 5, 46 (1993)].
It is also reported that the phase modulation efficiency of an n-i-P InGaAs/InP MQW ridge phase modulator at 1.52 μm wavelength is 12°/V·mm [U. Koren, T. L. Koch, H. Presting, and B. I. Miller, Appl. Phys. Lett., 50, 368 (1987)], and that of a P-n-i-n InGaAs/InP MQW ridge phase modulator at 1.55 μm wavelength is 39°/V·mm [H. K. Tsang, J. B. D. Soole, H. P. LeBlanc, R. Bhat, M. A. Koza, and I. H. White, Appl. Phys. Lett., 57, 2285 (1990)]. However, the phase change of the latter for the TE mode is non-linear and proportional to square of reverse bias voltage. Thus, switching operation is obtained at the bias voltage as high as 5V.
Compared with the phase modulators based on GaAs, the phase modulators based on InP generally have low phase modulation efficiencies, and thus the switching voltages are comparatively high. And, a P-n-i-n InGaAs/InP MQW ridge phase modulator has comparatively high phase modulation efficiency as described above (39°/V·mm at 1.55 μm wavelength), however, the phase change as a function of the bias voltage is non-linear and bias voltage of 5V is always required for switching operation. Therefore, it is required to develop an optical modulator having low switching voltage, in which the phase is being changed linearly with the applied voltage.